An optical fiber with a cover which may be used to measure the temperature of molten metal is disclosed in British Patent Application No. 1518208.2 filed on Oct. 14, 2015. The covered optical fiber is unwound from a spool and fed to the molten metal bath through a guide pipe. When a portion of the optical fiber is immersed into a molten metal at a predicable depth, the radiation light emitted from the molten metal at blackbody conditions is such that the intensity of the radiation using a photo-electric conversation element mounted on the opposite end of the immersed consumable optical fiber can be used to determine the temperature of the molten metal. During this measurement, the immersed portion of the optical fiber is consumed by the molten metal bath, such that continued temperature information is available only by providing a continued supply of fresh optical fiber.
Devitrificaton of a quartz optical fiber will result in an attenuation of the transmitted light, and thus result in an error proportional to the extent of this damage. It is well known in the art that the immersed optical fiber must be consumed at a rate which is equal to or faster than the rate of devitrification of the optical core in order to result in accurate performance. A variety of schemes to feed consumable optical fibers into molten metal all are designed to expose the optical fiber core to molten metal before its devitrification. However, the devitrification rate is dependent upon the structure of the optical cored wire and the actual conditions of the molten metal bath, such as its temperature, its fluid motion, the amount and type of slag covering the molten metal bath, as well as the thermal environment of the metallurgical vessel to which the optical fiber is exposed to before and after each measurement cycle. Multiple feeding schemes are likely to arise due to the numerous variety of conditions to which the optical fiber will be exposed during its introduction into and through various metallurgical vessels at various times during metals processing.
Thus, a problem to be solved is establishing a specific method to feed optical cored wire into a molten metal bath that is practical, that can be applied to a variety of molten metal vessels, and which takes into account the degradation of the optical fiber before, during and after its use especially when used for sequential measurements in a measuring sequence.
U.S. Pat. No. 5,585,914 discloses a single-metal, jacketed optical fiber that can be fed through a nozzle into a molten metal bath at a rate of 5 mm/sec for 10 seconds. The immersed fiber is then held in the immersed position for 20 seconds. When this method is carried out in a cyclic fashion, the method may be considered continuous. To obtain this type of operation, the single-metal, jacketed optical fiber is fed from a point below the metal surface through a nozzle in the sidewall of a vessel that requires a continuous gas shrouding of 5 Nm3/hour and at speed of 121 Nm/s. An advantage of this method is that the unfed optical fiber is kept cool by the gas shrouding. However, a problem associated with using the type of submersed access to molten metal, as taught by U.S. Pat. No. 5,585,914, is the ability to keep the nozzle open and free of obstruction. Once the opening is blocked, continued feeding is impossible. U.S. Pat. No. 8,038,344 discloses that additional pressure measurements should be concurrently employed with such gas purged nozzles in order to determine if the opening is blocked.
To circumvent this problem, the optical fiber can be fed into the molten metal bath from above its surface. However, this method is also not without some inherent problems. The optical fiber must transit the distance from the exit of a guide tube, through a cover of molten slag and then finally into the molten metal bath below the slag surface. In order to form the blackbody condition, which is a necessity for accurate measurements, the fiber must be immersed a minimum distance into the molten metal bath and to a location within the metallurgical vessel which is representative of the molten metal bath. During this time while the optical fiber is immersed in the molten metal bath, the metal jacket of the optical fiber is subject to radiant, convective and conductive heating. Any softening of the optical fiber can result in bending of the optical fiber out of the molten metal bath due to the buoyancy of the optical fiber, aided in some instances by fluid currents of the molten metal bath. Thus, in the harsh industrial environment of molten metal processing vessels, maintaining a predetermined depth of the optical fiber as is necessary to ensure blackbody conditions during the period of measurement, has proven to be difficult due to the inherent weakness in the prior art metal jacketed optical fiber as the temperature increases.
Multi-layered wires with steel outer coverings are used in steelworks to introduce additive substances selectively into the molten metal bath. For example, such wires are disclosed in DE 19916235 A1, DE 3712619 A1, DE 19623194 C1, and U.S. Pat. No. 6,770,366. U.S. Pat. No. 7,906,747 relates to repeatable exposure of the additive to the molten metal, particularly molten steel. The efficiency of adding these doping substances to the steel using a cored wired, a filled wire, or wire shaped additives, depends upon delivering the doping additive to a specific distance below the molten metal surface. This is accomplished by special machines and feeding methods that can payout a specific length of additive cored wire at a speed sufficient to guarantee that the destruction of the outer steel jacket, thus exposing the additive to the molten metal, will occur at a prescribed depth. Long lengths of cored wires are supplied in coils or on spools, for example as disclosed in U.S. Pat. No. 5,988,545, for integration with special wire feeding machines, for example as disclosed in EP 0806640 A2, JP H09101206A, JP 56052507A and DE 3707322 C1, in order to carry out the practical immersion of additive cored wires. The construction of optical cored wires and the used of cored wire feeding machines have benefited from the teaching of additive cored wires. However, this body of prior art is silent as how to control the immersion of an optical cored wire to expose the continuously consumable optical core to the molten metal while also addressing the devitrification rate of the optical fiber.
JP 09304185A discloses a feeding rate solution wherein the speed of fiber consumption must be greater than the rate of devitrification, thereby assuring that a fresh optical fiber surface is always available. New material is constantly fed to replace devitrified fiber and is thus suitable for receiving and passing on radiation, without radiation losses. Therefore, the method of feeding cannot be independent of the optical fiber structure itself. The optical fiber is sent out into the molten metal until a threshold of 1200° C. is achieved. The optical fiber is then stopped and the temperature is recorded. After a first period of 2 seconds, a fixed length of 10 mm of the optical fiber is fed into the molten metal bath and the temperature is again recorded. The second recorded temperature is then compared to the first recorded temperature. A comparison of the first and seconds temperatures determines if a successful measurement has been achieved or if additional cycles are needed.
Besides being a means to determine if the reading is acceptable, the speed of feeding is not specified. It has been found that in more harsh measuring environments, such as an electric arc furnace, the speed of feeding is a significant variable due to the amount of preheat that occurs above the molten metal before immersion of the optical fiber in the molten metal. In the case of multiple immersions, the thermal exposure of the optical wire occurs in the time interval between measurements. Substantial preheating by radiant exposure will promote devitrification which manifests itself in a lower than actual temperature. During metallurgical processing, the actual temperature could legitimately have decreased during the manufacturing process and not be related to the devitrification of the optical fiber. Thus, known methods are insufficient, because there is no separation between an actual change in the temperature due to the process and a change in the measured temperature due to devitrification. Additionally, the internal environment of a melting vessel could easily exceed the preset temperature even before immersion of the optical fiber. The rate of devitrification is the controlling factor for accurate temperature measurements, and thus feeding the optical fiber is a function of both the optical fiber construction and the environment to which it is exposed before, during and after its immersion.
Feeding methods that rely upon measuring a threshold temperature before actuation neglect the fact that after a reading, the remaining optical cored wire or metal jacketed optical fiber will become devitrified by heat conduction in the interval between the present immersion and the future immersion. Devitrification will lead to incorrect light gathering and therefore erroneous temperatures leading to improper feeding judgements. Therefore, in order to practice a method of optical wire feeding, the cored optical fiber remnant of the prior measurement must be considered so as not to influence the subsequent measurements. Some prior art has recognized this limitation. For example, JP H09243459A teaches a corrective action in that damaged immersible optical fibers should be cut away from the supply coil each time to provide an un-devitrified fiber. However, this reference provides no indications of how one is to determine the extent of devitrification. In practice, this also requires additional equipment to cut away the damaged portion of the fiber and, in the case where the immersion is from above the molten metal bath, the fiber must be withdrawn through a layer of slag. In turn, the slag may collect on the fiber, thereby interfering with the removal from the vessel and eventually the cutting mechanism.
U.S. Pat. No. 7,748,896 discloses an improved optical fiber device for measuring a parameter of a molten metal bath. The device comprises an optical fiber, a cover laterally surrounding the optical fiber, and a detector connected to the optical fiber, wherein the cover surrounds the optical fiber in a plurality of layers, one layer comprising a metal tube and an intermediate layer arranged beneath the metal tube. The intermediate layer comprises a powder or a fibrous or granular material, wherein the material of the intermediate layer surrounds the fiber in a plurality of pieces. The intermediate layer is formed of silicon dioxide powder or aluminum oxide powder and may contain a gas producing material.
A counterpart patent, U.S. Pat. No. 7,891,867, discloses a method of feeding such optical cored wires by determining an initial temperature response interval. The speed of fiber fed is determined by the change in detected temperature during a first thermal response interval compared to a change in detected temperature during a second following time interval. The speed of optical feeding is therefore optimized during feeding and adjustable by parameters that are independent of the structure of the optical fiber. Also, only the identification of the thermal response time within two time intervals are required. While adequate for spot measurements, this method of controlling the feeding rate fails to account for the fact that once begun, repeated measurements do not exhibit an initial thermal response interval as described. According to this prior art method, the heating rate and hence the thermal response are a result of the feeding speed, slag temperatures as well as melt temperature of the particular furnace. However, the optical fiber will receive radiation before it is immersed into the metal. The very low temperatures of the first interval described in this prior art method, in most circumstances will occur out of the molten metal bath, and therefore the first interval is not a characteristic of the optical fiber response to the metal but rather to that of the thermal conditions of the melting furnace.
Thus, there is a need for a predictable immersion methodology that is suitable for a first immersion and then repeated immersions that do not depend upon a cool down period between them or achievement of a starting threshold.
It is well known to those skilled in the art that the bath level within the melting furnace is subject to variation due to the contour and wear of the refractory lining of the vessel. This presents a problem in that the installation position is mostly fixed to the physical structure of the vessel, while the ideal immersion depth and location within the bath are mostly variable. Since the rate of devitrification, as previously described, is a function of the amount of heat input to the optical core before, during and after its molten metal immersion, the multiple heat sources of the application environment must also be considered to be variable since the distances that the optical cored wire must travel to become immersed will vary according to each vessel geometry and refining process.
Thus, there is a need for a simple yet effective means to control the feeding of improved optical cored wire that brings an un-devitrified optical fiber to a measuring location that is capable of an initial single immersion and closely followed multiple immersions for use in metallurgical vessels, especially electric-arc furnaces.